Such a mill can be, for example, a ball mill, or else an SAG (semi autogeneously grinding) mill that is intended for milling coarse grained materials such as, for example, ores or cement etc. In the case of such mills, the current filling level in the drum in which the comminution takes place is normally unknown. Specifically, the filling level depends on many variables. Examples of these are the exact degree of milling, the proportion of balls that are introduced into the drum to assist the milling operation, the degree of wear of these balls, and the proportion of solids in the suspension that is currently located in the drum. These variables change for the most part during operation of the mill. Their current values are unknown in the same way as is the value of the filling level itself.
A somewhat accurate knowledge of the current filling level would also be very advantageous since it would be possible to derive conclusions therefrom regarding the efficiency of the mill operation. In the case of overfilled mills, the comminution is inefficient owing to the small dropping height and the energy absorption of the already comminuted milling stock. In the case of underfilled mills, the drum walls and the drivers can be damaged. The speed of the drum can be better set with the aid of the current filling level and, if appropriate, further parameters such as the hardness of the stock or the proportion of solids to be milled.
At present, the filling level is estimated by the operating staff using its empirical values. Weight sensors that determine the applied weight of the loaded drum on the bearings are used by way of support. Despite these additionally provided sensors, this estimation method is very inaccurate. Acoustic measuring methods have also recently been developed, but these likewise require additional sensors for receiving sound.
Conventional methods for acquiring filling levels such as, for example, the rotating vane, pendulum and vibration measuring methods offered by Mollet Füllstandstechnik GmbH by means of the website http://www.mollet-gmbh.de/ are suitable, rather, for stationary storage containers but not for a rotating and loaded drum of a mill.